Cetyltrimethylammonium bromide (CTAB) induces partially irreversible compaction of DNA-adsorbed layers on hydrophobic silica. Additionally, there is a synergistic increase in the adsorbed amount when both CTAB and DNA are present as compared to the surface excess concentration of either of the individual components. In this study of the DNA adsorption and DNA-CTAB coadsorption by ellipsometry, emphasis has been placed on the DNA molecular weight as well as its conformation (single and double stranded). The DNA molecular weight and conformation have a large effect on the surfactant-free DNA adsorption behavior but not on the mixed DNA-CTAB adsorption behavior. Comparison between interfacial and bulk complexation has been made where possible. The DNA-CTAB complexes at the interface are neutral despite the large excess of DNA in the bulk. The final structure of the adsorbed layer was found to be dependent on the history of complex formation and DNA size.